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Science : Hope dawns for spinal cord repair

THE regeneration of damaged spinal cords, long considered one of the most
elusive goals in neurobiology, is no longer a dream but a realistic target,
thanks to research by scientists in Sweden.

Henrich Cheng and his colleagues at the Karolinska Institute in Stockholm
have shown that rats whose spinal cords had been completely severed can be made
to use their legs again within months. Using tissue grafts and doses of a
growth
factor, the researchers encouraged enough nerve cell regeneration for the
injured animals to regain some mobility (Science, vol 273, p 510).

“This study is a major milestone,” says Wise Young, professor of
neurosurgery
at New York University Medical Center, in a commentary published in the same
issue.

Once severed, spinal nerve cells never regrow the long fibres, called axons,
that innervate muscles elsewhere in the body. In part, this is because most of
the tissue of the spinal cord produces a chemical, neurite inhibitory factor
(NIF), which blocks axon growth.

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But in the last ten years, neurobiologists have shown that the axons of some
nerve cells will regrow long distances into nerve tissue outside the spine,
because this tissue lacks NIF. With this in mind, Cheng and his colleagues took
nerve fibres from the rats’ ribcages, which do not produce NIF, and
transplanted
them into the spine to bridge the gap where the spinal axons were severed. They
stabilised the connections with fibrin (a plasma protein that sets like glue),
and also provided a substance called fibroblast growth factor (FGF), which
stimulates nerve growth.

The transplanted nerve bundles formed tiny tunnels through which severed
spinal axons could regrow, even though the transplants never carried
impulses of
their own.

But if the axons grew straight across the gap, they would again encounter
spinal cord tissue with NIF and stop growing. So the researchers used the nerve
bridges to divert the growing axons away from their usual pathway and into
nerve
tissue elsewhere in the spinal cord that does not produce NIF. The axons then
presumably found their own way to the leg muscles.

Within three weeks, this procedure began to restore hind leg movement in
treated rats. The animals continued to improve for the full year of the
experiment, although none recovered fully.

Paralysed people should not raise their hopes unduly, Young warns. “Though
the Grail has been sighted, it is not yet in hand.” The rats in Cheng’s
experiments had cleanly cut spinal cords, whereas most people with spinal
damage
suffer crush injuries.

Other experts are also cautious. “The degree of functional recovery they
achieved is significant but not revolutionary,” says William Blakemore of the
University of Cambridge, who studies nerve damage. The researchers did not test
regeneration with FGF alone, so it is difficult to judge how effective the
bridge technique had been. Even so, he says, the work is “extremely
interesting”.

Stephen Bradshaw, director of external affairs for the Spinal Injuries
Association in London, raises another issue. “Not all the money for spinal
injuries should go into high-profile research of this type,” he says. “There’s
also a need for resources so that people living with spinal injuries now can
live happy, useful lives.”